234 THE ANATOMY OF INVERTEBRATED ANIMALS. 



.Z?, A), having a remote resemblance to the operculum of 

 Limulus. 



THE ENTOMOSTRACA. All the remaining Crustacea have 

 completely specialized jaws; and as many as six pairs of 

 appendages may be converted into gnathites. 



In the Entomostraca, if the body possesses an abdomen 

 (reckoning as such the somites which lie behind the genital 

 aperture), its somites are devoid of appendages. Moreover, 

 the somites, counting that which bears the eyes as the first, 

 are more or fewer than twenty. There are never more than 

 three pairs of gnathites. The embryo almost always leaves 

 the egg in the condition of a Nauplius ; that is, an oval 

 body, provided with two or three pairs of appendages, which 

 become converted into antennary organs and gnathites in the 

 adult. The division of the Entomostraca comprises the 

 Copepoda, the Epizoa, the Branchiopoda, the Ostracoda, 

 and the Pectostraca. 



THE COPEPODA. In these Entomostraca, which come 

 nearest to the Eurypterida, the cephalic shield, which is dis- 

 coidal and not folded longitudinally, is succeeded by a certain 

 number of free thoracic and abdominal somites. The anten- 

 n tiles and antennae are large, and, as in the Eurypterida, are 

 organs of locomotion and sometimes of prehension. The an- 

 terior thoracic members are converted into foot-jaws ; the 

 posterior serve as paddles, the limbs of each pair being often 

 united together in the middle line, as in Limulus. The em- 

 bryo leaves the egg as a Nauplius. 



The various species of the genus Cyclops, which abound 

 in fresh water, afford excellent illustrations of the structure of 

 the Copepoda. 



The minute animal (Fig. 60) is shaped something like a 

 split pear, the larger end corresponding with the head, and 

 the convex side with the dorsal surface. The anterior third 

 of the body is covered by a large carapace, which, at the sides, 

 extends downward as a free fold over the bases of the ap- 

 pendages, but is hardly at all free posteriorly. Anteriorly, 

 in the middle line, it curves forward and downward, and is 

 produced into a short rostrum, on each side of which a con- 

 siderable excavation lodges the base of the long antennule, 

 by the vigorous oar-like strokes of which the animal darts 

 through the water. At the anterior boundary of the head, 

 the double, black, median eye, which, unless very closely ex- 



